The BALB (BALB/cJ) and B6 (C57B1/6J) mouse strains vary greatly in several parallel HSC (hemopoietic stem cell) phenotypes. This project's overall objective is to understand HSC regulation by defining this range of normal variation and finding critical cellular and molecular points where regulation occurs. This may suggest improvements in clinical marrow transplantation. Aim 1 tests if the B6 advantage in self-renewal is due to better proliferation, better migration, or both. BALB or B6 HSC functions will be compared after either proliferation or migration in vivo. Aim 2 tests if the B6 advantage is intrinsic (cell autonomous) or extrinsic. BALB and B6 marrow will be transplanted into identical H-2 matched BALBB6F1 recipients, or allophenics will be made from BALB and B6 embryos. In both types of chimeras, B6 HSC will self-renew better if their advantage is intrinsic. Aim 3 tests if B6 and BALB strain-specific phenotypes in parallel HSC attributes in vivo - aging, self-renewal, developmental loss and number - are regulated by the same mechanisms. Strong correlations of BALB and B6 patterns in the 13 CXB RI lines would support this idea. Other phenotypes in cell markers and expression arrays will also be compared in the RI lines. Meaningful correlations will be confirmed in congenics, backcrosses and groups with chromosome recombinations. Aim 4 defines the genetics behind the relationships in aim 3. RI line patterns will suggest chromosome locations of genes regulating aging, self-renewal, and development. These will be confirmed using congenics and backcrosses. Progeny will be tested from males with chromosomal recombinations carrying unique sub-sets of BALB and B6 alleles narrowing the chromosome region in 2 stages to 1 cM and 0.1 cM. Candidate genes will be tested in transgenics and ultimately identified by homologous recombination.